The correct answer is: D. Air masses are created by patterns of heating and cooling of the Earth's surface.
Explanation:
The air masses can be simply be divided into warm and cold ones, further they can be moist or not. The formation of the air masses, be it warm and cold, over the land masses is because of patterns of heating and cooling. This results some areas to produce very large air masses that are cold and dry, like Siberia, or warm and dry large air masses, like Sahara. Also, on a local level there's air masses that develop every day and night, which is noticeable during the night and the morning, where the colder air masses from the mountains move down to the valleys, and the warm air masses from the valleys move up to the mountains. That is a because of the change in the heating and cooling patters that differ during the day and the night.
